There are three typical systems for ink-jet printing. In one system ink is ejected in the form of droplets through a small nozzle (measuring usually several microns to several hundreds microns in diameter), and to each droplet is imparted a predetermined static charge by electrical signals, followed by electrostatic deflection of the droplets between high-voltage deflection plates to form a character, a figure, or the like on a recording medium according to a predetermined dot matrix. In a second system ink is drawn in the form of droplets through a small nozzle of the above-specified size under the electrostatic control of high-voltage plates, accelerated, and then deflected electrostatically between deflection plates to form letters or figures on a recording medium according to a predetermined dot matrix. In a third system ink droplets are ejected in response to electrical signals to form letters or figures on a recording medium according to a predetermined dot matrix.
That is to say, typical ink-jet printing systems consist basically of forming a letter or a figure on a recording medium by ejecting ink droplets through a small nozzle, which are electrically charged and controlled electrostatically, or by discharging ink droplets from a small nozzle in response to electrical signals.
Aqueous ink used in these systems for ink-jet printing typically comprises water as a vehicle, a hydrophilic organic solvent, a coloring dye, and other optional additives. With such ink, letters, figures, or pictures can be printed on various objects, such as plastics, metals, paper, cloth, and so on.
For recording letters, figures, or pictures on such objects using an ink-jet printing process, the aqueous ink must meet the following requirements:
(1) Requirements for providing uniform ink droplets through a small nozzle: PA0 (2) Requirements for providing good color in an image on a recording medium; PA0 (3) Requirements for providing a good image on a recording medium:
(a) the ink should have a viscosity ranging from 1 to 10 cps at 20.degree. C.; PA1 (b) the ink should have a surface tension of more than 20 dynes/cm; and PA1 (c) the ink should be homogeneous and comprised of uniformly sized particles. PA1 (a) the ink should contain dyes that provide colors identical to those required in the final color image; PA1 (b) in the case of color reproduction according to the subtractive process, the ink preferably contains yellow, magenta, and cyan dyes which absorb light only in their respective desired spectral ranges, and desirably do not absorb light in other ranges; PA1 (c) the ink should provide a color image which is satisfactorily sharp, and does not spread on the recording medium; PA1 (d) the ink contains dyes that provide high chroma and color density on the recording medium, and does not change in hue during storage or after it is applied onto the recording medium to form a color image (that is, the ink should have excellent light-fastness); PA1 (e) the ink should contain dyes that have a water solubility high enough to provide an ink composition of high chroma and color density; PA1 (f) the ink should contain dyes that do not render the ink extremely viscous; and PA1 (g) the ink should contain dyes that have low crystallizability such that they do not crystallize or solidify during storage or on standing in the ink to form deposits that clog the nozzle. PA1 (a) the ink droplets applied onto the recording medium penetrate thereinto rapidly and dry quickly enough to form a letter or a figure that does not lose shape; PA1 (b) the ink upon drying has water resisting property high enough to provide that the letters or figures formed do not dissolve in water; and PA1 (c) the ink has good tack (adherence) to the recording medium.
It is known that water-soluble dyes which can be used in inks for ink-jet printing include certain of those described in Color Index,. published by The Society of Dyers and Colorists and the American Association of Textile Chemists and Colorists, as disclosed in published unexamined Japanese Patent Application Nos. 12105/72, 9905/76, 120678/80 and 46979/80. More specifically, as known examples of water-soluble dyes for the abovedescribed purpose, metnion may be made of those described in published unexamined Japanese Patent Application Nos. 146304/77, 115306/78, 89534/74, 43153/80, 53591/80, 145773/80, 78076/80, 50072/80, and 15622/75, and so on.
However, inks using these conventional dyes have somewhat unsatisfactory hue and low chroma, and the combination of yellow, magenta, and cyan ink compositions incorporating these dyes does not always provide a color image with faithful color reproduction. As a further disadvantage, ink using these dyes does not always have satisfactory characteristics in that it sometimes clogs the nozzle during continuous operation or when the operation is restarted after a short or a long suspension, or in that it provides an image that spreads on the recording medium.